1. Abstract Two-thirds of breast cancers express estrogen receptor alpha (ER1) and rely upon ER1 signaling for their growth. Estrogen deprivation is a powerful treatment for ER1 positive breast cancers. However, intrinsic and acquired resistance to endocrine therapy remains a significant cause of disease relapse and mortality in ER1 positive tumors. Epigenetic silencing of ER1 downstream target genes is recently identified as a critical factor in endocrine therapy resistance. For example, several recent findings confirmed the clinical and therapeutic significance of the epigenetic silencing of ER1 target genes like HOXB13. CD24 is gaining significant attention in breast cancer research in connection with its negativity in breast tumor initiation, metastasis, and as a potential breast cancer stem cell (BCSC) marker. Though CD24 is expressed in primary tumors, several, studies have shown that CD24 negative cells are highly tumorigenic and its over- expression in negative breast cancer cells results in the loss of metastatic potential. This suggests that the loss of CD24 expression may be critical for gaining metastatic potential. Interestingly, epithelial cells with the CD44 positive CD24 low/absent marker combination (ESA????/low) are considered putative BCSCs. We have previously published that the high prevalence of BCSCs in primary tumors favors distant metastasis. Similarly epithelial mesenchymal transition (EMT) in human mammary epithelial cells resulted in CD24 negative phenotype with enriched tumor-initiating and metastatic potential. Our previous publication showed that estrogen represses CD24 transcription and ER1 binds to palindromic EREs in the CD24 promoter. A recent ONCOMINE analysis of 17 independent breast cancer studies suggested a strong inverse correlation between ER1 positivity and CD24 expression. This suggests the possibility that CD24 differential regulation is critical in estrogen-mediated signaling of primary breast tumors. How CD24 is getting silenced in breast tumors is an important and unexplored question. Our preliminary studies gave exciting evidence that CD24 is epigenetically silenced in breast tumors by promoter hypermethylation. To date there has been no data available regarding how frequently and specifically CD24 is silenced in breast tumors compared to the surrounding normal tissues. Also its hormonal influence as well as clinical and therapeutic significance is not known. Before performing expensive and labor intensive larger epidemiological studies, it is necessary to create reliable preliminary data on the frequency of CD24 silencing in breast cancer and its correlation to ER1 expression. This pilot study will compare the frequency of CD24 methylation in 40 ER1-positive and 40 negative breast tumors and their pathologically normal surrounding tissues. Successful completion of this pilot study will provide critical preliminary data for our future extensive epidemiological study. We have an excellent team of collaborators including experts in breast cancer, epidemiology, epigenetics, and biostatistics, who are excited to work with us. PUBLIC HEALTH RELEVANCE: Project Narrative Endocrine therapy like Tamoxifen treatment remains important in premenopausal women with hormone receptor positive breast cancer as it work best in women whose tumors are positive for estrogen receptor (ER1) and unfortunately it is estimated that one third of women treated with Tamoxifen for 5 years will have a recurrence within 15 years suggesting these are endocrine resistant tumors which represents about one-fourth of all breast cancers. Silencing of hormone regulated genes in tumors are known to cause this resistance and this pilot project will focus on the prevalence of the silencing of one of the hormone target genes (CD24) in breast tumors compared to the surrounding normal tissues and its correlation to hormone receptor status. Results from this pilot study will provide important data to plan future larger studies to better understand the role of this silencing in therapeutic resistance and clinical outcome.